This book contains a description and analysis of the spatial and temporal variations in the Arctic and Antarctic sea ice covers from October 26, 1978 througb August 20, 1987. It is based on data collected by tbe Scanning Multichannel Microwave Radiometer (SMMR) onboard the NASA Nimbus 7 satellite. The 8.8-year period, together with the 4 years of the Nimbus 5 Electrically Scanning Microwave Radiometer (ESMR) observations presented in two earlier volumes ( Antarctic Sea Ice, 1973-1976: Satellite Passive-Microwave Observations and Arctic Sea Ice, 1973-1976: Satellite Passive-Microwave Observations) comprises a sea ice record spanning almost 15 years. The sea ice measurements presented in this volume represent a distinct advance over the Nimbus 5 data used in the earlier books. This advance results from the improved characteristics of the SMMR, compared to the ESMR: .ESMR was a single-frequency, singly polarized sensor; SMMR was a multifrequency, dual-polarized sensor. .ESMR had cross-track scanning and a varying incidence angle; SMMR had conical scanning and a constant incidence angle. The multifrequency, dual-polarized constant incidence-angle characteristics of the SMMR have permitted .A more accurate calculation of total sea ice concentrations (fraction of ocean area covered by sea ice ). .Determination, for the first time, of both multiyear sea ice concentrations and the physical temperatures of the sea ice. .An overall accuracy of ~7% for the SMMR total ice concentrations. This compares with estimated accuracies of ~25% in the Arctic and ~15% in the Antarctic for the ESMR. .More accurate measurements of the sea ice extents (areas enclosed by 15% ice-concentration boundaries), sea ice areas, and open water areas within the ice margins. SMMR measures vertically and horizontally polarized radiances at five wavelengths, ranging from 0.81 cm to 4.5 cm, and provides a total of ten channels of radiance data. The spatial resolution of the SMMR varles with wavelength and ranges from 30 to 150 km. The 0.8- and 1.7-cm channels are used for calculating ice concentrations and have integrated fields of view of 30 and 55 km, respectively. In this volume, the 55-km spatial resolution of the SMMR ice concentrations is maintained by mapping the SMMR radiances onto a polar stereographic projection with a grid spacing of about 25 km. A description of the sea ice cover is provided by a variety of maps and plots: .Monthly averaged maps of total sea ice concentrations produced from SMMR data for the Arctic and Antarctic for each month from November 1978 through August 1987. .The 8- or 9-year averages by month over the SMMR lifetime. .Monthly anomalies with respect to these averages. .Time series of the ice extents, the areal coverage by ice, and the open water within the ice-ocean boundary . .Maps of monthly averaged multiyear sea ice concentrations, along with their long-term averages and anomalies (Arctic only). .Maps of monthly averaged sea ice temperatures, with the additional use of the vertically polarized 4.6-cm channel. For the purpose of examining regional differences, the Arctic is divided into nine regions and the Antarctic into five. We provide .Time-series analyses for each region and the hemispheric and global totals. .Selected single-day Arctic ice-concentration maps to illustrate the rapid changes in the sea ice distributions. Analysis of the Arctic SMMR time series for the nine regions combined reveals that the Arctic seasonal cycle ranges, on average, from a minimum of 9 x 106 km2 in September to a maximum of 16 x 106 km2 in March. The Arctic Ocean contributes about 14% of the 7 x 106 km2 seasonal range and the balance is contributed by the perimeter seas. The interannual variability of the ice extent areas is much larger for the perimeter seas than for the Arctic as a whole; some regions exhibit decreasing trends, while others exhibit increasing trends. Negative trends in the Sea of Okhotsk, the Greenland Sea, and the Kara and Barents Seas are countered in part by positive trends in the Arctic Ocean, Bering Sea, Hudson Bay, and Baffin Bay, Davis Strait, and Labrador Sea, resulting in a small but statistica1ly significant negative trend ( -2.1~O.9% ) over the 8.8-year record. Ice-free areas within the sea ice pack are also found to have a negative trend ( -3.5~2.0% ). The confidence level for each of these trends is better than 90%. The 8.8-year SMMR record also reveals that the Arctic Ocean ice pack at the height of the summer melt season usually assumes one of two different positions over the years of SMMR coverage: .The Siberian mode, characterized by the ice pack being distant from the Alaskan coastline, leaving extensive open water areas along its shoreline. .The Alaskan mode, characterized by the ice pack impinging on or being close to the Alaskan coastline. A relationship between this bimodal behavior of the summer ice distribution and the general oceanic circulation patterns in the Arctic Ocean can be inferred from the Arctic Ocean Buoy Program (AOBP) data: .The Siberian mode occurs when the general circulation pattern assumes its long-term state, i.e., with a well-developed Beaufort Gyre and Transpolar Drift Stream. .The Alaskan mode occurs when the long-term state breaks down. These two modes are evenly divided over the 1979-1987 time span (1981 had an ambiguous ice distribution). This equal division contrasts with the earlier ESMR years when the Siberian mode persisted for 3 of the 4 years. The higher accuracy of the SMMR ice concentrations has increased the confidence in the observation of areas of reduced concentration (polynyas ) that occur in the Arctic Ocean from November through April. These winter polynyas have typical lifetimes on the order of months; the summer polynyas persist days or weeks. The areas, or zones, where polynyas occur most often are the southern Beaufort Sea (adjacent to the McKenzie Delta and Alaskan coast), near the center of the usual location of the Beaufort Gyre, and along the Siberian sector of the Transpolar Drift Stream. These winter polynyas do not form with any regularity in any zone. The ability to determine multiyear sea ice concentration in winter leads to .Removal of some ambiguities in calculating the multiyear ice distribution within the central Arctic during the winter months. .An estimate of the accuracy of the derived multiyear ice concentrations of almost 11 %, almost twice the error in the total ice-concentration calculations. .The observation that multiyear ice concentrations in winter are as high as 96%-100% north of the Canadian Archipelago. .The observation of lower and more variable concentrations in other regions of the central Arctic, where the average concentration is about 60%. .The ability to analyze large-scale motions of the Arctic ice pack. .The observation that widely different distributions of the multiyear ice trom one year to the next are related to the bimodal nature of the summertime location of the Arctic ice pack. .Evidence that multiyear ice flows trom the Arctic Ocean into the channels of the Canadian Archipelago during winter, at rates consistent with independent observations at the surface and from Landsat images. There is a difference between the area of multiyear ice in midwinter and the total ice area in the previous September. The SMMR record indicates that only about 3 x 106 km2 of the 5 x 106 km2 ice area at the mid-September ice minimum becomes multiyear-ice. Most of the difference is explained by the fact that ice in mid-September includes not only ice that becomes multiyear ice the following winter, but also new ice that persists into the winter as first-year ice and some ice that will melt prior to the winter season. Since the entire ice pack does not make the transition from melting to freezing simultaneously, the ice present at the September ice minimum is not a good estimate of the multiyear ice area the following winter- In the Antarctic, the most dissimilar observation in the SMMR lifetime as compared to the ESMR is the absence of the Weddell Polynya, which had persisted throughout 3 of the 4 austral winters during the ESMR lifetime. The range in the SMMR Antarctic ice extents is about 16 x 106 km2, with a seasonal minimum of 3".5 x 106 km2 and a seasonal maximum of 19 x 106 km2. As in the Arctic, the individual sectors have larger interannual differences than in the Antarctic as a whole, implying compensating relationships in the various regions. For instance, a 7-year undulation in the sea ice extent maxima of the Weddell, Amundsen, and Bellingshausen Seas is countered by a similar but out-of-phase undulation in the Western Pacific. Although the ice extents in the Weddell Sea have a large downward trend during the 8.8-year SMMR record, this negative trend is countered by positive trends in the Ross Sea and Indian Ocean. The Antarctic ice extents as a whole have no statistically significant trend. Globally, the combined Arctic and Antarctic sea ice extent varies from a minimum of about 20 x 106 km2 to a maximum of about 30 x 106 km2 and is roughly in phase with the Antarctic oscillation. Thus, the average global oceanic albedo has an appreciable seasonal varlation in spite ofthe out-of- phase characteristic of the boreal and austral seasons. The 2.1% negative trend in the Arctic sea ice extent, combined with the lack of a trend in the Antarctic, results in an overall negative trend of about 1% in global sea ice during the SMMR lifetime. Monthly averaged ice temperatures derived from the SMMR data are given in Appendix A, which also includes coincident data from the AOBP. While not validated, the SMMR sea ice temperature data provide information not available from any other source, and may be potentially useful for surface-atmosphere heat transfer studies in the polar regions. The SMMR and AOBP temperature data sets have important differences in their interpretation. The AOBP temperatures are point measurements that approximate surface-air temperatures in winter. In contrast, the SMMR ice temperatures are a spatially averaged measure of the physical temperature of the radiating portion of the ice, typical of the snow-ice interface for first-year ice and of a weighted-mean temperature of the freeboard layer for multiyear ice. A linear regression analysis of the wintertime SMMR and AOBP temperatures shows the SMMR temperatures to be 5 K to 13 K warmer, on average, with the larger biases over regions of thick multiyear ice. The standard error of estimate between the two data sets is 4.7 K. This volume serves as a summary of the sea ice parameters derived from the 8.8-year SMMR polar data set. It is a companion to a series of 12 compact disk-read only memories (CD-ROMs) containing the SMMR polar data set. These CD-ROMs have been made available by the NASN Goddard Space Flight Center (GSPC) in Greenbelt, Maryland, and are archived at and distributed by the National Oceanic and Atmospheric Administration (NOAA) World Data Center/National Snow and Ice Data Center (Nsmc) in Boulder, Colorado. The CD-ROMs contain brightness temperature and ice concentration maps for every other day, all on the same polar stereographic grids used in this hook. The radiance data are for eight of the ten horizontally and vertically polarized channels, with the 1.4-cm channels not included because of severe instrument drift. An additional CD-ROM is planned that will include the monthly averaged ice concentrations and ice temperatures as they appear in this book, along with tables of values for the curves. Similar brightness temperature and sea ice concentration maps are being created and distributed on CD-ROMs hy Nsmc for the Defense Meteorological Satellite Program Special Sensor Microwave/ Imager (SSMI), launched on June 19, 1987. Por the purpose of providing a consistent set of sea ice parameters, a comparison was undertaken that shows that concentration differences (SMMR-SSMI) during the 2 months of overlap are 0.2%:t5% during austral winter and 0.5%:t5% during boreal summer. This measure of consistency between the two data sets should encourage the use of the combined data sets, wbich provide well over a decade of continuous global, multispectral passive-microwave observations. |
